3. •The only difference of this circuit is that the outgoing
Thyristor keeps conducting till the incoming thyristor is
triggered.
•Each device conducts for 120
•Each phase supplies power for 120.
•A positive voltage across the blocking thyristor is possible.
•Continuous output voltage(0<α<30°)
•Discontinuous output voltage (30°<α<150°)
3-phase Half-wave controlled rectifier with R load
4. Three Phase Fully Controlled Bridge Converter
with R Load
•T1, T3, T5 : Common
Cathode Configuration
•T2, T4, T6 : Common
Anode Configuration
•If all are diodes,
changeover takes place
at X,Y,Z and P,Q,R
•T5 T1 takes place at X
•T6 T2 takes place at P
6. Three Phase Fully Controlled Bridge Converter
with R-L-E Load
For any current to flow in the load at least
one device from the top group (T1, T3, T5)
and one from the bottom group (T2, T4,
T6) must conduct.
Thyristors are fired in the sequence
T1 → T2 → T3 → T4 → T5 → T6 → T1 with
60° interval between each firing.
Thyristors on the same phase leg are fired at an interval of 180° and
hence can not conduct simultaneously.
This leaves only six possible conduction mode for the converter in the
continuous conduction mode T1T2, T2T3, T3T4
, T4T5, T5T6, T6T1.
8. Three Phase Fully Controlled Bridge Converter
with R Load
• =0 is equivalent to an uncontrolled full
wave bridge
•Thyristors should be fired at X,Y Z and
P,Q,R
•Each device conducts for 120
•Each phase supplies power for 240 in a
cycle provided load current is continuous.
•6 Pulses per Cycle, each of 60 .
•At any time only two phase supply power
to the load and one phase is open.
11. Three Phase Fully Controlled Bridge
Converter with R Load ( = 60)
For what firing angle,, the current through the load
becomes discontinuous ?
For =0 Vo varies from 1.5 V to 1.732 Volts
For =30 Vo varies from 0.866 V to 1.732 Volts
For =60 Vo varies from 0 V to 1.5 Volts
At =60, Vo touches the X-axis, so as the load current.
So beyond =60 load current becomes discontinuous.
13. Three Phase Fully Controlled Bridge
Converter with R Load ( = 90)
For =90 Average value of the output voltage,Vo ,=0 but
the current is continuous. When is this possible?
This is possible only if the load is purely inductive. (The
average value of the voltage across an inductor is 0).
For other loads a continuous conduction is not possible for
=90.
14. Three Phase Fully Controlled Bridge
Converter with R Load ( = 120)
15. Three Phase Fully Controlled Bridge
Converter with R Load ( = 150)
16. Converter & Inverter
Converter Operation
For =0 Vo varies from 1.5 V to 1.732 Volts
For =30 Vo varies from 0.866 V to 1.732 Volts
For =60 Vo varies from 0 V to 1.5 Volts
For =90 Vo varies from -0.866 V to 0.866 Volts
Inverter Operation
For =120 Vo varies from -0.866 V to -1.5 Volts
For =150 Vo varies from -0.866 V to -1.732 Volts
17. Question
A DC motor (R-L-E Load) speed need to be controlled
using a single phase fully controlled converter. What
should be the values of at starting and later to increase
speed?
The natural answer is =90 and then keep on
decreasing.
At =90, the instantaneous value of the input is maximum
and if this is applied to the armature, a very high current
will pass through the armature and damage.
So the safe value of is around =150and reduce it
slowly
18. Comparison of various converters
The filter requirement reduces with increase in number of pulses
per cycle.
